1. Field of the Invention
The present invention relates, in general, to a method for wet etching polysilicon and, more particularly, to an improvement in the etching selectivity between doped polysilicon and undoped polysilicon.
2. Description of the Prior Art
For fabrication of a semiconductor device, many etching process steps are efficiently undertaken, Of the etching process steps, wet etching, which utilizes a solution of chemical etchant, is the most frequently conducted.
A typical solution of chemical etchant for polysilicon consists mainly of nitric acid (HNO.sub.3) and fluoric acid (HF). When this chemical etchant is applied to a polysilicon film, the oxygen radicals of nitric acid combine with the silicon atoms to form a silicon oxide film, which is then etched by an action of fluoric acid.
The wet etching process is characterized by two functions: etch rate and selectivity. The etch rate of the chemical etchant for polysilicon is virtually determined by both the formation rate of an oxide film through the reaction of nitric acid with silicon and the removal rate of fluoric acid for the oxide film.
During the fabrication of a semiconductor device, a patterning process step comprising etching selectively of a doped polysilicon film, which is in a state of lamination with an undoped polysilicon film, is very frequently required. For example, when a pin structure of capacitor is formed, a process for patterning selectively doped polysilicon through etching is efficiently used.
However, the typical solution of chemical etchant is not satisfactory in etching selectivity between doped polysilicon and undoped polysilicon, and it cannot remove the doped polysilicon as precisely as necessary.